The present invention relates to an apparatus for detecting contaminants depositing on the reticle of an exposure system such as a reduction projection mask aligner, photo-repeater, and 1-to-1 exposure system.
First, the principle of the reduction projection mask aligner, as a typical exposure system, will be described with reference to FIG. 1. This exposure system uses a mercury lamp 1 to produce the illumination light 50. The light beam from the mercury lamp 1 goes through a condenser lens 2, an interference filter 15, a diaphragm 16, a mirror 3, and a condenser lens 4, and illuminates a reticle 5 on a photographic plate provided with an LSI pattern 5a. The image of the pattern 5a is reducibly focused on the surface of a wafer 8 by a contracting lens 7. (The reduced projection is shown by the solid line.) The wafer 8 is coated with photoresist, and the contracted image of the LSI pattern 5a turns to a latent image in the photoresist. Upon exposure by the illumination light 50 for the preset exposure time controlled by a shutter 51, the wafer 8 is taken out of the system and developed, then a contracted pattern derived from the LSI pattern 5a is obtained on the wafer.
In this case, if contaminants 6 exist on the reticle 5, they cause a faulty LSI pattern image on the wafer and such a wafer is abandoned.
If contaminants 6 deposit on the reticle 5 by some reason, they interrupt part of the illumination light to a chip on the wafer, and pin holes are created in the pattern. Since a number of chips on the wafer 8 are exposed automatically by moving the wafer in the X and Y directions by the step-and-repeat operation, contaminants 6 on the reticle 5 create pin holes on every chip and make all the chips useless. On this account, after the exposing and developing processes, photoresist patterns on the wafer 8 need to be inspected, and this visual inspection always overlooks some defects. Dust particles floating in the air of the workroom could deposit to the reticle 5 after being set in the reduction projection mask aligner. Therefore, the reticle 5 in the set position in the system must be checked for contaminants frequently, and if contaminants are detected during exposure, it must be alerted immediately, the exposing process must be suspended, and the reticle must be replaced.
However, in practice, there exist the condenser lens 4 and the reticle mount devices over the reticle 5, therefore the reticle 5 cannot be observed visually once it has been set in the system. Accordingly, it has been the tradition to take out the reticle 5 occasionally from the optical exposure system of the reduction projection mask aligner and perform external inspection. This method poses a problem of further adhesion of contaminants to the reticle during the transportation by the operator.
When the contraction lens 7 is assumed to have a contraction factor of 1/10 (such as a case of using Zeiss S-Planar 50 mm), a particle with a dimension of 10 .mu.m or larger adhering on the upper surface of the reticle or a particle with a dimension of 5 .mu.m or larger adhering to the bottom surface of the reticle can cause a faulty latent image, and such contaminants must be detected while the reticle is already placed in the system.
One prior art of this matter is employed in the reduction projection automatic mask aligner "DSW" manufactured by the U.S. firm, GCA, and this method will be described with reference to FIG. 2. First, a reticle 5 is transported automatically to the position below the condenser lens 4, while being checked for contaminants on the reticle 5 during the transportation. For the detecting operation, a laser source 9 is placed above the reticle 5, and the laser beam is focused on the surface of the reticle by a convergence lens 10. If contaminants 6 are put to the reticle 5 during the transportation, and come to the focal point of the laser beam, the laser beam will scatter. A lens 12 and a photoelectric detector 13 are provided nearby the focal point, and the presence of contaminants is discriminated in accordance with the output of the detector.
In such a system, contaminants are detected during the transportation of the reticle, and they cannot be detected after the reticle has been set completely under the condenser lens 4. In most cases, however, contaminants are put to the reticle after it has been set completely under the condenser lens, and therefore, the reticle must be checked occassionally in this state. The above-mentioned conventional method is not proper for this purpose, and an alternative method which solves the problem has been desired.